Maybe find a good designer that can provide services to install and setup the spaces.A lot of these ideas are going to depend on where you are buying.

Additionally, I collaborated with architects, engineers, and contractors to finalize the design and construction plans for a stackable duplex.

Additionally, I collaborated with architects, engineers, and contractors to finalize the design and construction plans for a stackable duplex.

A HELOC or loan on your primary could provide funding, but that is typically not designed for longer term funding - i.e. a down payment on a multifamily....

I do like real estate, I have a great eye for design, but would hate to have to evict someone—that would be my worst nightmare, so I’m not sure I’m cut out for being a landlord (although I would fully anticipate having a property manager)Is option #3 a far inferior option compared to keeping our expensive house in expensive city as a rental and only having to deal with one property that gives us great appreciation in 10 years ?

You need to do an independent review of the sold comps, DOM, and the finishes you need inside to get the ARV you see.3) When projects pass the paper test and pass the number tests, then you need a GC and an Agent or some kind of designer, who have already looked at all the comps, walk the space and put together a budget that works and that will get you the number.4) Now you need a good relationship with a money source. 

I am a former NASA robotics scientist... trust me when I say, it IS better because I designed it to be better!

When I internet search “residential definition” the first definition is ”designed for people to live in”.  

I'd find a mortgage broker who understands your strategy well enough to advise on products designed to help you scale. 

@Jamie Parker, most of what you'll see in the education space of REI/business is fluff and designed to take your money.